Antibodies have been coined the ?magic bullets? against many human diseases. However, there remains significant challenges in the engineering of antibodies targeting multi-pass membrane proteins, which encompass a large number of therapeutic targets such as cell surface receptors and the ion channel proteins. The difficulty in engineering binders to membrane proteins stems from the limitation of the current in vitro selection/panning technologies, such as phage display, which require highly purified target protein. Unfortunately, membrane proteins are often refractory to purification due to their dependence on the cell membrane for proper folding and activity. Currently, there is no effective in vitro technology for the discovery/engineering of binders to multi-pass membrane proteins. The overall goal of this study is to develop a novel technology ? SMURF (Simple proxiMity coUpled mRNA display) ? for engineering protein binders to protein targets on the cell surface, thus bypassing the need to purify the target protein. SMURF combines mRNA display with the proximity-assisted-DNA-assembly phenomenon and, unlike conventional panning in which all binders to a solid support are enriched, SMURF fosters the enrichment of binders only to a desired target protein on the cell surface. In Aim 1, we will demonstrate the SMURF principle using oligonucleotides and optimize the primer sequences. Aim 2 will establish the SMURF enrichment of a model protein in a mixture of non-target proteins in solution. Finally, in Aim 3, a model protein will be displayed on the mammalian cell surface and a library of binders will be screened to demonstrate and quantify the whole-cell SMURF enrichment efficiency. The successful completion of this study will establish a novel technology for facile discovery/engineering of binders to whole-cell-displayed membrane proteins and should greatly expand the repertoire of drug targets amenable to therapeutic intervention.